Re: [PATCH v5 2/3] KVM: x86/ioapic: Implement support for I/O APIC version 0x20 with EOIR

[Khushit Shah]

> On 29 Dec 2025, at 6:31 PM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> 
> Hm? IIUC kvm_lapic_advertise_suppress_eoi_broadcast() is true whenever
> userspace *hasn't* set KVM_X2APIC_DISABLE_SUPPRESS_EOI_BROADCAST
> (either userspace has explicitly *enabled* it instead, or userspace has
> done neither and we should preserve the legacy behaviour).

The legacy behaviour for "kvm_lapic_advertise_suppress_eoi_broadcast()" is:
- true for split IRQCHIP (userspace I/O APIC)
- false for in-kernel IRQCHIP

The in-kernel IRQCHIP case was "fixed" by commit 0bcc3fb95b97 ("KVM: lapic:
stop advertising DIRECTED_EOI when in-kernel IOAPIC is in use"), which made
it return false when IOAPIC is in-kernel.

With this series, in QUIRKED mode the function still returns !ioapic_in_kernel(),
preserving that exact legacy behavior. The I/O APIC version 0x20 (with EOIR)
is only used when userspace explicitly sets the ENABLE flag.

The comments in patch 1 explain this in more detail ;)